1. Field of the Invention
A certain aspect of the present invention relates to a multi-beam laser light-intensity control circuit and an optical scanning apparatus including the multi-beam laser light-intensity control circuit.
2. Description of the Related Art
An optical scanning apparatus, for example, a laser scanning apparatus, forms an image using a laser beam. Such an optical scanning apparatus typically includes a laser power control device having an automatic power control (APC) function to control the light intensity of a laser beam and thereby to obtain an image with a desired density.
The APC function monitors the light intensity of a laser beam emitted from a light source such as a laser diode by receiving the laser beam with a light-receiving element such as a photodiode, and performs feedback control on a drive current supplied to the light source such that the monitored light intensity of the laser beam matches a predetermined reference value.
Japanese Patent Application Publication No. 2001-24273 discloses a multi-beam laser scanning apparatus including a laser power control device having an APC function. The disclosed multi-beam laser scanning apparatus is described below.
FIG. 5 is a schematic diagram of a multi-beam laser scanning apparatus 50 disclosed in Japanese Patent Application Publication No. 2001-24273.
In the multi-beam laser scanning apparatus 50, as shown in FIG. 5, laser beams LB emitted from laser diodes (LD) 51A and 51B are collimated by a collimator lens 53, shaped by a cylindrical lens 54, and projected onto a polygon mirror 55 being rotated at high speed. The laser beams LB are reflected by reflective surfaces of the polygon mirror 55 and are thereby deflected in the main-scanning direction. Then, the deflected laser beams LB pass through an f θ lens 56 and scan a photosensitive surface of a photosensitive drum 57 in the main-scanning direction.
The photosensitive drum 57 rotates around a rotational shaft 57a extending in the main-scanning direction and as a result, the photosensitive surface of the photosensitive drum 57 is scanned by the laser beams LB also in the sub-scanning direction. The laser beams LB are modulated according to image data so that a desired image or a desired pattern is drawn.
A synchronization photodiode 58 is disposed near one end of the photosensitive drum 57. The synchronization photodiode 58 receives the laser beams LB before they scan the photosensitive drum 57 in the main-scanning direction, and outputs horizontal synchronization signals used as timing signals for the main scanning and the APC control. Meanwhile, a part of each of the laser beams LB emitted from the laser diodes (LD) 51A and 51B is detected by a light-receiving element (photodiode) 52. Based on the detection results from the light-receiving element 52, a laser power control circuit (APC circuit) 59 controls the output power of the laser diodes (LD) 51A and 51B before the photosensitive drum 57 is scanned in the main-scanning direction.
FIG. 6 is a circuit diagram of a related-art laser diode driving device 30 (corresponding to the laser power control circuit (APC circuit) 59 of FIG. 5). In FIG. 6, it is assumed that the number of laser beams (or laser diodes) is two.
As shown in FIG. 6, the laser diode driving device 30 includes an automatic light-intensity control circuit 300. The automatic light-intensity control circuit 300 includes a PD control circuit A 301a, a sample-and-hold (S/H) circuit 1 302 including an error amplifying circuit A 3021 and a hold circuit A 3022, and a drive current generating circuit A 304 for automatic power control (APC) of a laser diode LD1; and a PD control circuit B 301b, a sample-and-hold (S/H) circuit 2 303 including an error amplifying circuit B 3031 and a hold circuit B 3032, and a drive current generating circuit B 305 for APC of a laser diode LD2.
Each of the PD control circuit A 301a and the PD control circuit B 301b converts a feedback current ID, which is generated by a light-receiving element PD and corresponds to the light intensity of a laser beam emitted from the laser diode LD1 (channel 1) or the laser diode LD2 (channel 2), into a feedback voltage VD.
The feedback current ID from the light-receiving element PD flows through a resistor Rpd1 or a resister Rpd2. Each of the resistors Rpd1 and Rpd2 causes a voltage drop and thereby generates the feedback voltage VD. Using variable resistors as the resistors Rpd1 and Rpd2 makes it possible to perform voltage conversion according to laser diode characteristics of the laser diode LD1 and the laser diode LD2 (the channels 1 and 2).
The error amplifying circuit A 3021 and the error amplifying circuit B 3031, respectively, compare feedback voltages VD obtained by the PD control circuit A 301a and the PD control circuit B 301b with predetermined reference voltages Vref1 and Vref2. For example, the reference voltages Vref1 and Vref2 are set at values that are equal to feedback voltages VD corresponding to the light intensities of the laser diode LD1 and the laser diode LD2 under normal operating conditions.
Each of the hold circuit A 3022 and the hold circuit B 3032 holds a difference voltage between the feedback voltage VD and the reference voltage Vref1 or Vref2. After APC is completed, the difference voltages are held to stably maintain the controlled state.
The drive current generating circuit A 304 and the drive current generating circuit B 305, respectively, increase or decrease LD drive currents based on positive or negative voltages held in the hold circuit A 3022 and the hold circuit B 3032.
Thus, feedback control is performed on the drive currents of the laser diodes LD1 and LD2 and the light intensities of the laser diodes LD1 and LD2 are thereby controlled to match the reference voltages Vref1 and Vref2.
In FIG. 6, enable signals XAPCEN1 and XAPCEN2 enable APC functions for the corresponding laser diodes LD1 and LD2. Image pattern data DATA1 and DATA2 are used to modulate the output power of the laser diodes LD1 and LD2 to form a desired image on a photosensitive drum (the photosensitive drum 57 of FIG. 5). In other words, the drive currents of the laser diodes LD1 and LD2 are turned on and off according to the image pattern data DATA1 and DATA2. The enable signals and the image pattern data are, for example, input from an external computer.
FIG. 7 is a flowchart showing a process performed by the laser power control circuit (APC circuit) 59 of FIG. 5. Here, it is assumed that the laser power control circuit (APC circuit) 59 has a configuration corresponding to the laser diode driving device 30 of FIG. 6.
Since operations of two channels corresponding to the laser diodes LD1 and LD2 are substantially the same, circuits related to the channel of the laser diode LD1 are used in the descriptions below and circuits related to the channel of the laser diode LD2 are shown in parentheses.
When the laser diode LD1 (the laser diode LD2) is turned on (step S101), the light-receiving element PD receives the laser beam emitted from the laser diode LD1 (the laser diode LD2) and outputs an electric current, i.e., a feedback current ID, corresponding to the received laser beam (steps S102 and S103).
The feedback current ID is converted into a feedback voltage VD by the PD control circuit A 301a (the PD control circuit B 301b) (step S104), and the feedback voltage VD is compared with the first reference voltage Vref1 (the second reference voltage Vref2) by the error amplifying circuit A 3021 (the error amplifying circuit B 3031) of the S/H circuit 1 302 (the S/H circuit 2 303) (step S105).
The error amplifying circuit A 3021 (the error amplifying circuit B 3031) determines whether the feedback voltage VD is lower than the reference voltage Vref1 (the reference voltage Vref2). When the feedback voltage VD is lower than the reference voltage Vref1 (the reference voltage Vref2) (YES in step S105), the difference voltage represented by “reference voltage Vref1 (or reference voltage Vref2)—feedback voltage VD” becomes a positive voltage and the positive voltage is held in the hold circuit A 3022 (the hold circuit B 3032) (step S106). Then, the drive current generating circuit A 304 (the drive current generating circuit B 305) is controlled based on the positive voltage (value) to increase the LD drive current (step S107). As a result, the output power of the laser diode LD1 (the laser diode LD2) increases, the feedback current ID from the light-receiving element PD increases, and the difference voltage between the feedback voltage VD and the reference voltage Vref1 (the reference voltage Vref2) decreases.
When the feedback voltage VD is higher than the reference voltage Vref1 (the reference voltage Vref2) (NO in step S105), the difference voltage represented by “reference voltage Vref1 (or reference voltage Vref2)−feedback voltage VD” becomes a negative voltage and the negative voltage is held in the hold circuit A 3022 (the hold circuit B 3032) (step S108). Then, the drive current generating circuit A 304 (the drive current generating circuit B 305) is controlled based on the negative voltage (value) to decrease the LD drive current (step S109). As a result, the output power of the laser diode LD1 (the laser diode LD2) decreases, the feedback current ID from the light-receiving element PD decreases, and the difference voltage between the feedback voltage VD and the reference voltage Vref1 (the reference voltage Vref2) decreases.
By repeating the above steps, the difference between the reference voltage and the feedback voltage becomes substantially zero and the output power of the laser diode LD1 (the laser diode LD2) matches the reference voltage Vref1 (the reference voltage Vref2). Through the above process, automatic power control (APC) is performed for the laser diode LD1 (the laser diode LD2).
With a related-art multi-beam laser scanning apparatus as described above, an image can be formed at high speed by using multiple laser beams emitted from laser diodes (semiconductor lasers) at once. Such a related-art multi-beam laser scanning apparatus includes a laser power control device (circuit), and the laser power control device includes, separately for each laser diode, a photodiode (light-receiving element) for monitoring light intensity and an APC function including a sample-and-hold circuit.
Meanwhile, in a multi-beam laser diode that is a collection of multiple laser diodes (semiconductor lasers) integrated in a package, the number of photodiodes for monitoring light intensity is generally one, or smaller than the number of laser diodes (see, for example, Japanese Patent Application Publication No. 2001-024273). However, in the multi-beam laser diode, an APC function including an error amplifying circuit is provided separately for each laser diode to control the light intensity.
Thus, in the related art, an error amplifying circuit is provided separately for each laser diode. Such a configuration increases the mounting area (or the size) and the costs of a laser power control device.